<p><em>underscore</em>, <em>asterisk</em>, <em>one two</em>, <em>three four</em>, <em>a</em>, <em>b</em></p>
<p><strong>strong</strong> and <em>em</em> and <strong>strong</strong> and <em>em</em></p>
<p><em>line
line
line</em></p>
<p>this_is_not_an_emphasis</p>
<p>an empty emphasis __ ** is not an emphasis</p>
<p>*mixed *<em>double and</em> single asterisk** spans</p>